1. Field of the Invention
This invention relates to exposure devices for use in optical instruments such as cameras or observation instruments.
2. Description of the Related Art
For ordinary cameras, the exposure device such as shutter and diaphragm to be used therein is available in many forms corresponding to the photographic characteristics or the like. Yet, no exposure device suited to the still video camera which necessitates far higher an accuracy of exposure control has been developed.
Of the conventional, publicly known exposure devices, what is so-called a diaphragm-cum-shutter has features adaptable to be used in the still video camera. But, the conventional, publicly known diaphragm-cum-shutter, because of its having the following drawback, has been difficult to use in the still video camera.
Most of the conventional diaphragm-cum-shutters are constructed with the use of a sub-diaphragm arranged in front of the light receiving element to open and close at the same time when the diaphragm of the photographic lens opens and closes, that is, in the form of a diaphragm-cum-shutter with sub-diaphragm. In operating this diaphragm-cum-shutter with sub-diaphragm, as an exposure operation goes on, at the same time that the diaphragm is opened, the sub-diaphragm also is opened. Since the sub-diaphragm has been opened, light enters the photosensitive element arranged behind the sub-diaphragm. The light that has entered the photosensitive element is integrated to a value which is detected as an amount of light incident on the image sensing plane. When that integrated value has reached a predetermined value (the exposure value determined depending on the result of light measurement), a current is supplied to an electromagnet for controlling the closing operation of the diaphragm. Thus, the diaphragm is closed.
This publicly known diaphragm-cum-shutter with sub-diaphragm has the feature that in the photographic situation where the brightness is so high that the diaphragm should be opened to a minimum aperture, even if, soon after the light having passed through the sub-diaphragm has entered the photosensitive element, the diaphragm closing signal is output to the electromagnet, the diaphragm, because of the response delay of the electromagnet, cannot immediately close and, therefore, an over-exposure always results.
Meanwhile, for the photographic situation of middle brightness level where the diaphragm does not fully open, the ratio of the amount of exposure during the opening operation of the diaphragm to the amount of exposure during the closing operation of the diaphragm is almost constant, permitting premature production of the diaphragm closing signal to the electromagnet by previously estimating the amount of exposure to be made during the closing operation of the diaphragm. Therefore, good linearity of exposure control can be assured. This leads to the possibility of controlling the exposure with very high accuracy.
However, for a photographic situation of lower brightness level than that at which the diaphragm fully opens, because the ratio of the exposure integrated until the diaphragm closing signal is given to the electromagnet to the exposure integrated during the closing operation of the diaphragm does not become constant, the amount of exposure during the closing operation of the diaphragm cannot previously be estimated. Hence, there are many occasions that an under-exposure or over-exposure is caused to make.
As a result, with the publicly known diaphragm-cum-shutter with sub-diaphragm, the exposure cannot be controlled in linear manner over the entire range of the high brightness region, the middle brightness region and the low brightness region. Therefore, fluctuations take place in the exposure value over the aforesaid three regions. For this reason, the publicly known diaphragm-cum-shutter with sub-diaphragm could not be used as the exposure device for the camera which requires high accuracy of exposure such as the still video camera.
So, to remove the above-described drawback of the publicly known diaphragm-cum-shutter with sub-diaphragm, use of an encoder for detecting the amount of movement of the diaphragm, or a diaphragm-cum-shutter with encoder, is proposed. This diaphragm-cum-shutter with encoder is arranged so that the amount of movement, or the position, of the diaphragm is detected by the encoder to determine what position the diaphragm takes or what size the exposure aperture has, at every one time unit, thereby giving an advantage that in best timing based on that result, the diaphragm closing signal can be applied to the electromagnet.
However, this diaphragm-cum-shutter with encoder, too, because of the following reason, is not suited to perform a highly accurate exposure control. In more detail, the running characteristic of the diaphragm at the time of the opening operation varies with environment such as temperature, humidity, the lattitude, aging variation, of the diaphragm, etc. Therefore, while the amount of opening movement, or the position, of the diaphragm can be determined, the running characteristics (the speed and acceleration at any given moment) of the diaphragm are left unknown. Hence, however accurate the detection of the amount of opening movement, or the position, of the diaphragm may be, the position, or the time at which the diaphragm starts to close cannot accurately be controlled. Also, in a case where a motor or like drive source is used as arranged to give the diaphragm a movement controlled in the opening direction, the variation of the drive voltage of the motor, the variation of the resistance value of the motor coil due to the elevation of the motor temperature by the current supply, and other factors cause the running characteristics of the diaphragm to vary. Therefore, even if the amount of opening movement, or the position, of the diaphragm has accurately been detected by the encoder, the position at which the diaphragm starts to close cannot accurately be controlled, because the running characteristics of the diaphragm are not detected. Hence, in the case using the encoder in determining the position of the diaphragm at a time during the opening operation of the diaphragm so that based on this determined value, the electromagnet is actuated to close the diaphragm, for fast and slow opening speeds of the diaphragm, even though their values determined by the encoder are equal to each other, the positions at which the diaphragm starts to close differ from each other. Therefore, the exposure times and the exposure quantities are caused to differ.
FIG. 5 is a graph illustrating different manners in which the diaphragm-cum-shutter of the type described above in which the opening movement of the diaphragm blades are driven by the motor, where the exposure quantity varies with variation of the running characteristics of the diaphragm blades due to the causes such as that of varying the drive voltage of the motor. In FIG. 5, what is represented by solid lines L is the ideal running characteristic. In the case of too fast an opening speed of the diaphragm blades, the running characteristic is shown by dashed lines L.sub.1. In the case of too slow an opening speed of the diaphragm blades, the running characteristic is shown by double dot and single dash lines L.sub.2. Also, in FIG. 5, P.sub.1 to P.sub.3 represent points of time at which a signal for closing the diaphragm blades is given to the electromagnet in the aforesaid cases respectively. P.sub.1 ' to P.sub.3 ' represent points of time at which the electromagnet actually starts to operate and also represent a size of aperture opening at each of these times.
As is apparent from FIG. 5, it is understood that when the opening speed of the diaphragm blades varies, the exposure quantity integrated during one cycle of opening and closing operation of the diaphragm blades is caused also to vary. Hence, the conventional diaphragm-cum-shutter with encoder which is characterized in that it is only the amount of opening movement of the diaphragm blades that the encoder is used to detect, has been unable to make a sufficiently highly accurate exposure control.